For Democrats railing against the Senate’s threat to mothball President Obama’s nominees to replace the late Justice Antonin Scalia, Republicans have three words: You started it.

From the ruthless attacks on Judge Robert Bork to the 28-month filibuster of Miguel Estrada and the nine other filibusters of judicial nominees by President George W. Bush, conservatives say Senate Democrats have led the charge in an increasingly partisan confirmation process.

“Harry Reid, Joe Biden, President Obama and their fellow Democrats are the ones who broke the process in the first place, and if they don’t like that the GOP is holding up the process, they’re just reaping what they sowed,” said Mark Hemingway in a Wednesday op-ed in The Weekly Standard.

Vice President Joseph R. Biden is often credited with ushering in the newly hostile climate when, as chairman of the Senate Committee on the Judiciary, he presided over the then-unprecedented savaging of Judge Robert Bork, President Ronald Reagan’s 1987 nominee to the Supreme Court, during confirmation hearings.

A few years later, Mr. Estrada’s nomination to the U.S. Circuit Court of Appeals was blocked for 28 months by Senate Democrats before he finally withdrew from consideration in 2003. An internal memo by Senate Majority Whip Richard J. Durbin’s staff unearthed shortly thereafter said Mr. Estrada was “especially dangerous” because “he is Latino.”

Senate Republicans have been criticized for holding up Mr. Obama’s nominees to lower courts since taking over the chamber a year ago, but it was then-Senate Majority Leader Reid who detonated in 2013 the so-called “nuclear option” that allows judicial and executive nominees to be approved or disapproved with a simple majority vote.

Even Mr. Obama played a role in preserving the system when, as a member of the Senate, he voted to filibuster Supreme Court nominee Samuel Anthony Alito Jr. in 2006.

The White House said Wednesday that President Obama now “regrets” his filibuster, but insisted the situation was different than Republicans’ current vow to block any nominee to replace the late Justice Antonin Scalia.

“These are two different things,” said White House press secretary Josh Earnest. “The president’s objections to then-President Bush’s nominee were based in substance.”

Senate Majority Leader Mitch McConnell announced shortly after Mr. Scalia’s death Saturday that the Senate would not consider a nominee until after the November election, saying that “the American people should have a voice in the selection of their next Supreme Court Justice.”

His statement prompted an outcry from Democrats, led by Mr. Reid, who said it would be “unprecedented” for the GOP to delay confirmation proceedings for a year.

“Republicans should not insult the American people’s intelligence by pretending there is historical precedent for what they are about to do. There is not,” said Mr. Reid in a Monday op-ed.

Democrats have pointed to the 1988 confirmation of Justice Anthony M. Kennedy to rebut the Republican assertion that no Supreme Court nominee has been approved in a presidential election year since 1880. But conservatives say that’s the exception that proves the rule.

Even though Mr. Kennedy was approved February 3, 1988, the process began eight months earlier with Mr. Bork’s nomination on July 1, 1987, or 16 months before the November 1988 election.

The Democrat-controlled Senate rejected the nomination 58-42 on Oct. 23, 1987. A second nominee, Judge Douglas Ginsburg, withdrew Nov. 7 in a brouhaha over whether he had smoked pot.

“The vacancy was lengthy to allow the constitutional process to play itself out, the original nominee was blocked, and the Senate confirmed Justice Kennedy unanimously on February 3rd,” said American Center for Law and Justice chief counsel Jay Sekulow in a statement.

“We are already past that point in this presidential election year, at a historic moment in which the Supreme Court’s ideological composition has never been more consequential,” he said.

Mr. Reid also argues that Republicans have a “constitutional duty to give a fair and timely hearing and a floor vote to the president’s Supreme Court nominees,” which conservatives dispute.

“The Constitution is framed to require the President to take many executive actions, but imposes virtually no legislative duties on the Congress (beyond paying salaries, receiving the state of the union, and a few other items),” said Josh Blackman, associate professor of law at the South Texas College of Law, on his popular legal blog.

“There is no textual or historical requirement to ‘consent’ to a nominee, and ‘advice’ could be to tell the President to pick someone else,” wrote Mr. Blackman. “The failure to vote to confirm a nominee is a political problem, not a constitutional problem. And one that must be assessed by the electorate.”

Sen. Ted Cruz, Texas Republican, decried Tuesday what he described as the “hypocrisy” of Democrats such as Sen. Charles E. Schumer of New York, who criticized Mr. McConnell’s stance against considering nominations until after the election.

“When you go right off the president and say, I don’t care who he nominates, I am going to oppose him, that’s not going to fly,” Mr. Schumer said Sunday on ABC’s “This Week.”

In 2007, however, Mr. Schumer said the Senate should block any Supreme Court nominations by President George W. Bush in his final year because the court was “already dangerously out of balance.”

Mr. Cruz, a candidate for the GOP presidential nomination, called for making the 2016 race “a referendum on the U.S. Supreme Court.”

“We should not allow a lame-duck president to ram through a Supreme Court justice that would dramatically alter the balance of power on the Court,” Mr. Cruz said on the Hugh Hewitt radio show. “Instead, we should make this Election 2016 a referendum on the U.S. Supreme Court. And if the Democrats want to put a hard-core liberal on the court, their avenue for trying to do so is win the election in November, but let the American people have a choice.”